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Different Types of Busbar 2013-06-21

Busbars can be sub-divided into the following categories, with individual busbar systems in many cases being constructed from several different types: a) Air insulated with open phase conductors b) Air insulated with segregating barriers between conductors of different phases. c) Totally enclosed but having the construction as those for (a) and (b) d) Air insulated where each phase is fully isolated from its adjacent phase(s) by an earthed enclosure. These are usually called 'Isolated Phase Busbars'. e) Force-cooled busbar systems constructed as (a) to (d) but using air, water, etc. as the cooling medium under forced conditions (fan, pump, etc.). f) Gas insulated busbars. These are usually constructed as type (e) but use a gas other than air such as SF6, (sulphur hexafluoride). g) Totally enclosed busbars using compound or oil as the insulation medium. The type of busbar system selected for a specific duty is determined by requirements of voltage, current, frequency, electrical safety, reliability, short-circuit currents and environmental considerations. Table 1 outlines how these factors apply to the design of busbars in electricity generation and industrial processes.

Choice of Busbar Material

At the present time the only two commercially available materials suitable for conductor purposes are copper and aluminium. The table below gives a comparison of some of their properties. It can be seen that for conductivity andstrength, high conductivity copper is superior to aluminium. The only disadvantage of copper is its density; for a given current and temperature rise, an aluminium conductor would be lighter, even though its cross-section would be larger. In enclosed systems however, space considerations are of greater importance than weight. Even in open-air systems the weight of the busbars, which are supported at intervals, is not necessarily the decisive factor.

The ability of copper to absorb the heavy electromagnetic and thermal stresses generated by overload conditions also gives a considerable factor of safety. Other factors, such as the cost of frequent supports for the relatively limp aluminium, and the greater cost of insulation of the larger surface area, must be considered when evaluating the materials. From published creep data, it can be seen that high conductivity aluminium exhibits evidence of significant creep at ambient temperature if heavily stressed. At the same stress, a similar rate of creep is only shown by high conductivity copper at a temperature of 150°C, which is above the usual operating temperature of busbars.